Copper brazing is an established method for joining conventional carbon steels. The copper braze is metallurgically compatible with the carbon steels and has a melting temperature that is lower than the carbon steels, so that upon heating to the melting temperature of the braze alloy, the copper braze flows into the desired joint region by capillary action and solidifies upon cooling so as to produce a leak-proof, high integrity bond between the components.
However, when using conventional low carbon steels such as AISI 1010, there is a shortcoming associated with the use of copper brazing. (AISI is the designation for the American Iron and Steel Institute.) The relatively high temperatures associated with the copper brazing process, about 1100.degree. C. to about 1120.degree. C., produce a coarse, strain-free grain structure of low strength in the final brazed, low carbon steel component. The copper brazed joint is generally characterized by a yield strength of only about 10,000 pounds per square inch (10,000 psi or 10 ksi). Accordingly, copper brazed assemblies are rarely used in demanding applications requiring high strength.
An exception to this generalization is the use of a copper brazed pump assembly within an automotive automatic transmission system. A copper brazed torque converter pump assembly is commonly employed in high performance applications wherein the pump is cycled at high revolutions per minute (RPM) and/or in diesel truck transmission systems which require high torque outputs. Copper brazing the blades to the pump housing within the torque converter pump, produces reinforcement and increases the rigidity and strength of the pump assembly, as compared to the practice of mechanically staking the blades in place without the subsequent copper brazing operation. Blades staked at a single position do not contribute to the rigidity of the pump assembly.
Unfortunately, the actual strength of the material used to form the torque converter pump housing, i.e., the AISI 1010 steel, is greatly reduced by the high temperature brazing cycle The strength of the pump housing is an important characteristic since increased strength allows the pump to be operated at higher RPM levels without excessive plastic deformation of the pump housing. Accordingly, high strength retention following the copper brazing process is a critical requirement, particularly when considering the intended high performance requirements for future torque converter pump assemblies which will include operating at these much higher RPM levels.
To achieve higher strengths for the pump housing, various other steels that are alloyed with elements which contribute to strength following brazing, have been substituted for the low carbon steels generally employed In particular, commercially available microalloyed high-strength low-alloy (HSLA) steels, such as compositions typical of grades AISI 050XF, or higher strength grades such as AISI 080XF, have exhibited higher strengths after thermal exposure to a simulated copper brazing cycle, as compared to the AISI 1010 steel. However, the initial high strengths of these HSLA steels are problematic in that the materials are difficult to form prior to the brazing cycle, particularly when using the tooling developed for the conventional low strength carbon steels.
Therefore what is needed is a material for use as a torque converter pump housing that is characterized by sufficient ductility so as to be readily formed and machined prior to the high temperature copper brazing operations, but that is also characterized by relatively high strength after such a brazing operation.